The present invention relates to a non-invasive pulse oximetry intrauterine sensor.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have been provided with two sets of light sources and photodetectors that are adapted to operate at different wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Known non-invasive sensors include devices that are secured to a portion of the body, such as a finger, ear or the scalp. In animals and humans, the tissue of these body portions is perfused with blood and the tissue surface is readily accessible to the sensor.
It is desirable that photoelectric pulse oximetry also be useful for monitoring the blood flow characteristics and constituents of a fetus. For example, monitoring fetal oxygen levels provides an effective way to detect and provide indications for treating hypoxia in the fetus during labor. However, known sensors adapted for use on infants or adults are not suited for intrauterine placement.
The environment in which the non-invasive intrauterine sensor must operate is fluid-filled (e.g., by amniotic fluid) and is only accessible through the restricted opening of the cervix. Visual inspection of the fetus and the sensor is likewise restricted. Moreover, the operating environment presents certain variants that interfere with detection of the fetal blood flow characteristics using known pulse oximetry techniques. For example, the presence of the waxy vernix caseosa, hair, mucus, blood and dead tissue cells on top of the fetal tissue surface against which the sensor is to be positioned create a problem in establishing contact between the optical components of the sensor and the surface of blood-perfused tissue. Detection of fetal blood flow characteristics by pulse oximetry is particularly complicated by the relatively low perfusion and low oxygen saturation of blood in fetal tissue. These environmental factors prevent known sensors from providing reliable information needed to calculate fetal blood characteristics.
It is known that positive attachment of a sensor to the tissue surface improves the quality of the photoelectric signal provided by the sensor. Positive attachment to a human's tissue may be obtained by vacuum, adhesives, tapes or devices such as clothespin-type clips. However, fetal tissue is relatively moist and there is limited access to the tissue surface. Consequently, conventional adhesives or tapes or clips are not adapted for intrauterine use.
Known techniques involving invasive attachment to fetal tissue, such as by a screw attachment penetrating the tissue, creates a risk that the fetus will suffer an infection or disfigurement. Non-invasive attachment, such as by vacuum, may also result in disfigurement if the sensor includes sharp surfaces that press into the fetal tissue surface, or if the sensor is attached to the fetal tissue surface with excessive force (e.g., heavy vacuum suction).
Moreover, the intrauterine probe sensor must be safely and reliably deliverable to the point of contact with the fetus. It is desirable that intrauterine fetal monitoring be available early in labor, for example, to detect and treat hypoxia in the fetus during labor. Contact with the fetus can be made after natural rupture of the amniotic membrane by manually inserting a probe sensor into the uterus from the vagina, but access to the fetus through the vaginal canal is restricted by the cervix, which may be only slightly dilated to one or two centimeters when the membrane ruptures. Thus there is need for a fetal probe sensor that can be delivered to the fetus through a slightly dilated cervix, and a delivery system for doing so safely and reliably.
The present invention is directed to measurement of the fetal blood flow characteristics using a probe sensor adapted for intrauterine placement. The sensor can be adapted to operate in accordance with the photoelectric pulse oximetry measuring techniques described above as well as to accomplish other measurement techniques for monitoring the well-being of the fetus. For example, it is well known that electrical heart activity corresponding to the heartbeat can be monitored externally and characterized by the electrocardiogram ("ECG") waveform. The present invention contemplates that the ECG waveform of the fetus can be measured by providing the probe sensor with an ECG electrode which is in electrical contact with the fetus when the probe sensor is in place in the uterus. Further, the present invention contemplates that maternal blood flow characteristics and the maternal ECG waveform also can be measured by providing the probe sensor with one or more light sources, one or more photoelectric detectors and an ECG electrode directed toward the uterine wall.
The present invention also contemplates that the intrauterine probe sensor may include a thermistor to measure the temperature of the fetus, and a heat flux sensor to provide an indication of the adequacy of fetal tissue perfusion.
The present invention contemplates further that non-invasive positive attachment can be accomplished without disfigurement of the fetus by using a deformable probe sensor which is positively attached by creating a partial vacuum in a cavity formed in the probe to cause the probe to conform to the tissue surface of the fetus and to form a gasket-type seal with the fetal tissue surface.